Approximating cell geometry in a cellular transmission system

ABSTRACT

Approximating cell geometry in a cellular transmission system User equipment (UE 1 ) for use in a cellular transmission system comprising a processor configuration ( 6 ) to provide data corresponding to first and second parameters (a, b) for dimensional extents of the cell, and to select one of a plurality of different approximate geometrical configurations for the cell in dependence on the relationship between the values of said parameters. The selected cell approximation is then compared with the UE&#39;s current location to determine if a cell handover is to be made. The cell approximation technique is described in relation to a DVB-T network.

FIELD OF THE INVENTION

This invention relates to a method of approximating cell geometry in acellular transmission system and particularly but not exclusively toimproving cell handovers.

BACKGROUND

Cell handovers occur between cells in a wireless mobile communicationsnetwork when user equipment (UE) moves from the coverage area of onecell to another. It involves setting up new connections and releasing ormaintaining old connections to network cells as the user equipment (UE)moves from the coverage area of one cell to another. Typically, thecoverage area of neighbouring cells overlap, which leads to thepossibility of maintaining multiple cell connections and handing over byincreasing, reducing or maintaining the number of UE-network cellconnections. In 3G systems, “soft-handover” uses this technique.

Handover in wireless cellular systems is normally a three-phase process:(1) measurement—measurement criteria, measurement reports; (2) handoverdecision—algorithm parameters, handover criteria; and (3)execution—handover signalling, radio resource allocation. As an example,measurement may be on a near continuous basis e.g. sampled every 100 ms,the decision is assessed regularly e.g. every 5 seconds and handover isinfrequent, depending on the UE usage, e.g. on average every 20 minutes.

Handover execution is typically initiated as a result of a decisionbased on the measurement of certain criteria e.g. signal quality betweena base station (BS) for the cell concerned and the UE. There arecircumstances where the measurement process may take considerable timeor may not be feasible while receiving a service over a currentconnection. This may cause delays in data flow and result in lost datadue to the handover process. An example is in terrestrial videobroadcasting (DVB-T) systems, where a typical terminal has only one DVBreceiver front-end, which is not capable of multiplexed scanning betweenthe transmissions of adjacent cells whilst concurrently receiving a DVBtransmission, and so would have to temporarily cut the connection to thecurrent service to perform a multi-frequency scan for radio bearers andinterrogate control signalling on each available bearer signal todetermine if adjacent cells are suitable candidates for a handover.

A simple method to enable faster discovery of co-located and adjacentcells is for a UE to discover some of the connection parameters forthose cells in advance of physical measurement and data parsing of thecells concerned. For example, DVB describes a Network Information Table(NIT), which may define all the cells in a DVB network and includes datacorresponding to their frequencies and cell geography. The cells aredefined as a rectangle projected onto the spherical surface of theEarth, and the cell descriptors include cell id, cell latitude, celllongitude, cell extent of latitude and cell extent of longitude. Thecell latitude and longitude may define the southwest corner of therectangular cell and the extents of latitude and longitude define thelengths of side edges of the rectangle extending from the southwest cellcorner.

A problem with this configuration is that the rectangular definition ofthe cell is a poor approximation of its actual transmission coveragearea, which degrades the handover process.

SUMMARY OF THE INVENTION

According to the invention there is provided user equipment for use in acellular transmission system, comprising a processor configuration toprovide data corresponding to first and second circular parameters forthe dimensional extent of at least one cell of the system.

The processor configuration may operable to provide the data as afunction of major and minor axial extents of an ellipse and the data maycorrespond to characteristics of relatively large and small circles,which may be concentric. Furthermore, the processor configuration may beoperable to provide data corresponding to the centers of the circles.

The processor may be operable to select one of a plurality of differentapproximate geometrical configurations for the cell in dependence on therelationship between the values of said parameters.

The user equipment may be supplied with information corresponding to arectangular approximation of the cell, such as DVB-T NIT information andthe processor configuration may operable to convert information intosaid data. This may involve converting the NIT information into aCartesian reference frame.

The user equipment may comprise a mobile device operable to receive DVBtransmissions and may be further operable as telecommunicationsapparatus.

Circuitry to provide data corresponding to the current location of theuser equipment may be provided, which may be compared with the datacorresponding to the cell for determining whether a cell handover is tobe carried out.

The invention further provides user equipment for use in a cellulartransmission system, comprising a processor configuration to providedata corresponding to first and second parameters for dimensionalextents of the cell, and to select one of a plurality of differentapproximate geometrical configurations for the cell in dependence on therelationship between the values of said parameters.

The invention also includes a corresponding method, and a network thatmakes use of the inventive method.

The invention improves the accuracy of cell approximation and alsoprovides an arrangement which improves the cell handover process.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood an embodimentthereof will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic block diagram of a mobile network;

FIG. 2 is a schematic block diagram of a UE for use in the network ofFIG. 1;

FIG. 3 is an illustration of a cell coverage area with a rectangular andelliptical cell approximation;

FIG. 4 illustrates the elliptical cell approximation for an originatingcell and a target cell;

FIGS. 5 a and 5 b illustrate different elliptical configurations inwhich the large circle and the small circle or the rectangle constitutea better match for the cell concerned;

FIG. 6 is a flow diagram for a handover process performed by the UE;

FIG. 7 is a more detailed block diagram of the cell model selection; and

FIG. 8 is a more detailed block diagram of the process for determiningwhether the UE is within the coverage area of the target cell.

DETAILED DESCRIPTION

System Overview

In the following description, the invention is described by of examplewith reference to a DVB-T network although as will be evidenthereinafter it can be applied to any cellular network. Referring to FIG.1, UE1 is a dual mode mobile device for use with a UMTStelecommunications network and the DVB-T network. The UMTS networkincludes a base station BS1 connected to a land network 2, although inpractice, the UMTS network will include many base stations and only oneis shown in order to simplify the drawing. The UMTS network providescellular voice and IP data communication with UE1 in a manner known perse.

The DVB-T network includes geographically spaced apart base stations T0and T1 that may be connected in a network to a content source 3 shownschematically that can supply UE1 with video streaming or other data.The data received through the DVB network may be used in conjunctionwith services provided through the UMTS network. The network may includemore than the two base stations shown in FIG. 1 and each of themprovides a cellular area of coverage. Furthermore, each base station maysupport one or more cells and each cell may be supported by more thanone base station (not shown in the figure).

The device UE1 may be configured in a number of different forms asalready proposed in the art.

In this example, UE1 comprises a combined mobile telephone handset andpersonal digital assistant (PDA). A schematic block diagram of the UE1is shown in FIG. 2. For UMTS operation, the device includes an antenna 4a coupled to UMTS transceiver circuits 5 that are coupled to acontroller 6 that comprises a digital processor. Bi-directional voiceand data communication can be performed through the UMTS network. Moreparticularly, voice signals for transmission can be developed throughmicrophone 7 and received audio signals can be fed to a loudspeaker orearpiece 8. Telephony dialling and data manipulation can be carried outby means of a keyboard 9 or other communication interface like voiceresponse unit (not shown). Data can be displayed on a display device 10,which may be a LCD.

Also, data from the DVB network may be transmitted to on a downlink UE1and received by antenna 4 b, parsed by DVB-T circuits 11 and fed tocontroller 6 for display on the display device 10. Audio signals fromthe DVB transmission may be fed to the loudspeaker 8. The DVB system mayalso have a relatively narrow band uplink channel and data maytransmitted through the circuits 11 and antenna 4 b to one of the DVBbase stations T0, T1. The controller 6 has an associated data store 12which may comprise an EEPROM that can store the previously discussed DVBNIT data concerning the cell geography, when transmitted on thedownlink.

The UE1 may also include satellite global positioning (GPS) circuitry 13coupled to the controller 6, in order to determine its latitude andlongitude using signals from GPS satellites

Each of the DVB base stations T0, T1 has a transmission range 14, 15shown schematically in FIG. 1, of the order of 20-60 km and theircoverage areas partially overlap. In this example, UE1 is incommunication with the DVB cell provided by base station T0 and is inthe area of overlap with the cell of base station T1, and so it may bedesirable to make a cell handover.

Elliptical Cell Approximation

The shape of the cell coverage areas will now be considered in moredetail with reference to FIG. 3. Although in theory the coverage area ofa cell is circular, in practice it is of a non-regular shape, caused byhills, buildings and other obstructions within the vicinity of the basestation antenna. This is illustrated schematically in hatched outline 14for the cell provided by base station T0. A corresponding rectangularcell approximation 16 is shown, which may correspond to the conventionalNIT data for the cell concerned.

In accordance with an embodiment of the invention it has beenappreciated that using circular parameters and considering the cell interms of an ellipse 17 can achieve a better approximation. In manycircumstances, an ellipse can provide a better fit to the actual cellgeometry than a rectangle, as shown in FIG. 3. However, when consideringa cell handover in terms of overlapping elliptical cells, themathematics involved for computing overlying ellipses is much morecomplicated than for rectangular cell approximations and may not besuitable for a mobile UE with limited processing power. In accordancewith an embodiment of the invention, an improved simplification for anelliptical cell has been devised. The ellipse 17 is characterised interms of a large circle A of radius a drawn on its major axis and asmall circle B of radius b drawn on its minor axis, the circles beingconcentric in this example and centered on a point m, n in a Cartesianreference frame originating at 0,0 latitude and longitude. Thus, thecell can be defined in the following notation: {m, n, a, b}.

This nomenclature is developed in FIG. 4 for two cells labelled 0 and 1for a situation when a handover is to be considered, i.e. from cell 0referred to as the originating cell, to cell 1, referred to as a targetcell, i.e. from cell {m₀, n₀, a₀, b₀} to cell {m₁, n₁, a₁, b₁}. In FIG.4, the notation is simplified by centering the co-ordinate system on thetarget cell i.e. target cell′={0,0,a₁, b₁} and originating cell′={m₀-m₁,n₀-n₁, a₀, b₀}={m₀′, n₀′, a₁, b₀}. (The ′ symbol is used to denote thissimplification).

Best Fit

In accordance an embodiment with the invention, the cell approximationfor an individual cell may be selected on the basis of an individual oneof the circles of radius a, b or on the basis of a rectangle ofdimensions 2a, 2b, the choice being made on the basis of which is thebest fit for the cell concerned. The best fitting cell geometryapproximation can be determined by an analysis of the relationship ofthe dimensional parameters a and b. This can be seen in general termsfrom FIGS. 5 a and 5 b. The ellipse of FIG. 5 a is better approximatedby one of the circles A, B whereas in FIG. 5 b, the cell is betterapproximated by the rectangle 16. A more formal mathematical analysiswill now be given. Hereinafter, the larger circle A=L, the smallercircle B=S, the ellipse 17 is referenced E and the rectangle 16 isreferenced R.

When the Rectangle R is a Worse Match than the Larger Circle L

From FIGS. 5 a and 5 b, it can be seen that L is sometimes a bettermatch to the ellipse E than the rectangle R and sometimes worse. Ingeneral, the smaller the ratio of the radii (a/b), the better L matchesthan R. Based on the premise that both L and R completely contain theellipse, it is possible to calculate the ratio at which they are anequal match as the areas will be equal.

Assuming a>b.

Circle area=π.a.a (π˜3.142)

Rectangle area=2a.2b

Thus 2a.2b=π.a.a, and b/a=π/4˜0.7854

Removing the assumption that a>b, either b/a=π/4 or a/b=π/4

So the larger circle L is a better match when b is between 78.5% and121.5% of a.

When the Rectangle R is a Worse Match than the Smaller Circle S

Also, as a/b becomes larger, R will become a better match than thesmaller circle S. This point can be found when the area of R that is notin the ellipse E, is equal to the area of the ellipse that is not in S.

Two ways to represent this are in terms of

(i) absolute areas and (ii) percentage of areas:

Considering the areas of the rectangle R, the ellipse E and the smallercircle S:

area(R)=2a.2b=4a.b

area(E)=π.a.b

area(S)=π.b.b (assuming a>b)

(i) Considering the aforementioned equality in terms of absolute areas:

area(R)−area(E)=area(E)−area(S)

i.e. 4a.b−π.a.b=π.a.b−π.b.b

a.b.(4−2π)=−ππ.b.b

a/b.(4/π−2)=−1 (dividing by π.b.b)

a/b=1/(2−4/π)˜b 1.3760

Removing the assumption that a>=b, either b/a or a/b=1/(2−4/π)

So, on the basis of absolute area, the smaller circle S is a bettermatch than the rectangle R when b is between 62.4% and 137.6% of a.

(ii) Considering the aforementioned equality in terms of ratio of areas:

(area(E)/area(R))=(area(S)/area(E))

(π.a.b)/(4.a.b)=(π.b.b)/(π.a.b)

π/4=b/a 4˜0.7854

Removing the assumption that a>b, either b/a π/4 or a/b=π/4

So, for area ratios, the smaller circle S is a better match when b isbetween 78.5% and 121.5% of a. This is the same result as for L and socan be used to simplify the number of comparisons needed to determinethe best fit.

From the foregoing, it will be understood that the ellipse model for theradio cell can be approximated according to a number of differentoptions, as follows:

1. Approximation to a Small Circle and a Large Circle

-   -   The smaller circle S is completely within the ellipse E and the        large circle L completely contains the ellipse. Thus, being        within the coverage area of the smaller circle guarantees that        the UE in the cell coverage, and being outside the coverage area        of the larger circle guarantees that the UE is outside the cell        coverage. The third possibility, that the UE fits the larger but        not the smaller area, provide a “maybe in cell coverage”        alternative, which may be employed.        2. Approximation to a Small Circle and a Large Rectangle    -   This is an optimization of the previous alternative where a        rectangle provides a better match than the larger circle. A new        term, L′, in introduced so that L′=R in this alternative and        L′=L in the previous option. Otherwise, the idea is the same as        the previous option.        3. Approximation to a Circle    -   This is effective when radius a is very similar to radius b. Can        be thought of as a sub-type of the first option where L=S.        Cell Handover

The general process for achieving a cell handover is shown in FIG. 6. Atstep S6.0 a selection of a model of the target cell is made. Thisprocess selects which of the cell approximation options listed above isto be used.

At step S6.1, the current location of UE1 is determined. This currentlocation is compared with the selected cell model at step S6.2 and ifthis indicates that the UE is within the operational range (step S6.3)of the target cell, the handover process is carried out at step S6.4.

Features of this overall process will now be described in more detail.

Cell Model Selection (Step S6.0)

The process for selecting the cell model (step S6.0) for use in thehandover may be performed at the UE by means of the controller 6 and isshown schematically in FIG. 7. At step S7.0, the UE receives dataconcerning the target cell. This may comprise NIT data in a DVB-Tnetwork. As previously discussed, the rectangular cell data comprisesthe latitude and longitude of a corner of the rectangular cell, and thelatitudinal and longitudinal extents of the cell.

These data are at step S7.1 converted by simple trigonometry out of theangular, latitudinal and longitudinal frame and manipulated to providethe parameters m, n corresponding to the center of the rectangular cellin a Cartesian reference frame, and also the dimensional parameters a,b. It will be understood that the radii a and b for the cell whenapproximated as the ellipse correspond to half the latitudinal andlongitudinal extents of the cell in the NIT cell data when convertedinto the Cartesian reference frame.

At step S7.2, the ratio a/b is computed. At step S7.3 a determination ismade of whether R or L is a better match for the cell concerned. Fromthe foregoing, it will be understood that L is better match if0.785<a/b>1.215. Also, if a/b≈1, the circle approximation may be used(option 3). At step S6.4 the data concerning the better match is storedfor future use i.e. L′=R or L.

Collecting Location Information (Step S6.1)

As previously explained, the UE needs information about its currentlocation so that this can be compared with the selected approximation ofthe cell coverage area of the target cell in order to determine whetherthe UE is within the target cell. The obtaining of the current locationinformation of the UE is shown at step S6.1 in FIG. 6 and will now bediscussed in more detail.

Four scenarios are possible depending on how much detail the UE has onits current location:

1. The UE knows its current location exactly and uses this (“exactly”includes some negligible tolerance error)

2. The UE knows its current location approximately and uses this(tolerance is non-negligible)

3. The UE approximates its current location to that of the center of theoriginating cell

4. The UE approximates its current location to the area of theoriginating cell

It is evident that cases 1 and 3 are similar and cases 2 and 4 aresimilar. Cases 1 and 3 shall be known as the “point” case and 2 and 4shall be known as the “area” case. Also, there are two sub-cases for thearea case where:

a. it is sufficient to know that a target cell overlaps with some of theoriginating cell (so that a list of “potentially” available cells can becollected)

b. it is necessary to ensure that the target cell completely overlapswith the originating cell (to ensure that the target cell coverageincludes the current UE (point) location.

In general, sub-case a is more likely as more cells are likely topartially overlap than completely overlap.

It should be noted that some use cases might employ a “close enough”requirement. For example, a UE in a car maybe traveling sufficientlyfast that it predicts a different current location and area based on itscurrent location (and area) and velocity (speed and direction). Theseapplications would use scenario 2 with the modified location parameters.

Several methods can be used to attain the location information. Theseinclude:

-   -   (i) Delivery.    -   The UE gets the location of the originating cell base station        which either signal its geographical location (as in DVB-SI)        and/or its cell id, which can be mapped to geographical        information from some other source (e.g. DVB-TPS mapping to        DVB-SI or to a URI). The delivery can be either announced        (unidirectional signaling from network) or interrogated        (bi-directional signaling between UE and network).    -   (ii) Triangulation.    -   The location of cell base stations is known, as in (i), and the        temporal delay or signal gain (loss) is measured between the UE        and at least three base stations and thus it is possible to use        trigonometry to estimate the current UE location. This may be        achieved for example by monitoring signals from the UMTS base        station BS1 shown in FIG. 1 and others (not shown) to carry out        the triangulation.    -   (iii) Positioning.    -   The GPS circuitry 13 shown in FIG. 2 gives the UE its location.

The method (i) is more suited to scenarios 3 and 4, and methods (ii) and(iii) are more suited to scenarios 1 and 2.

Determining Whether the UE is within the Coverage Area of the TargetCell (Step S6.2)

This will now be described for the DVB-T network of FIG. 1 and aschematic flow chart for the process performed by the controller 6 isshown in FIG. 8. In this example, it is assumed that the cell modelselection process (step S6.0) has selected the elliptical cell modelthat comprises the large and small circles L, S although it will beevident hereinafter that the process can be modified if a different cellmodel has been selected. The outcome of the process can be YES, MAYBE orNO. Also, only one target cell is discussed (i=1), but the algorithm canbe iterated at various points for multiple target cells (i=1 . . . n)for situations where the relative merits of handing over to one of anumber of target cells needs to be considered.

The process may be performed by the controller 6 of UE1 and commences atstep S8.0. The subsequent steps of the process will now be considered indetail.

Step S8.1. Determine the Target and Originating Area Parameters

-   -   This information may be derived from the DVB-T NIT data. The        data may be converted from angular latitude and longitude data        as previously described, into Cartesian frame location        information: {m_(i),n_(i),a_(i),b_(i)}. The process is performed        for the originating cell (i=0) and the target cell (1=1). Thus,    -   For i=0,1        -   If b_(i)>a_(i), then l_(i)=b_(i), L_(i)=B_(i), s_(i)=a_(i),            S_(i)=A_(i),        -   else; l_(i)=a_(i), L_(i)=A_(i), s_(i)=bi, S_(i)=B_(i),    -   Considering the location of the UE, in scenarios 1 and 3 point        case), a₀=b₀=0 i.e. the UE is considered to be at the center of        the originating cell, whereas in scenario 2 (UE area case), a₀        may be equal to b₀ and they represent the UE area (not the        originating cell)        Step S8.2. Determine the “Current Location” to Use (m_(o),n₀)    -   In scenarios 1 and 2, “current location” is the UE location        (center point)    -   In scenarios 3 and 4, “current location” is the originating cell        center        Step S8.3. Calculate the Distance (d) Between the Center of the        Target Cell and the Current Location    -   use Pythagoras: x²+y²=h²,    -   h is the distance, d    -   x is the horizontal distance, (m₁-m₀)    -   y is the vertical distance, (n₁-n₀)

In the point case (scenarios 1 & 3)

Step S8.4 a. Is the Current Location within the Area of the Target Cell?

-   -   if s₁>d or s₁=d then the result is YES (the point is within the        smaller target cell circle)    -   if l₁<d then the result is NO (the point is not within the        larger target cell circle)*    -   otherwise, the result is MAYBE (the point falls between the two        target cell circles)

In the area case (scenarios 2 & 4), sub-case “a” (some overlap—seeprevious discussion)

Step S8.4 b. Is the Current Location Area Overlapping the Area of theTarget Cell?

-   -   if d<s₁+s₀ then the result is YES (the smaller circles overlap)    -   if d>l₁+l₀ then the result is NO (the larger circles do not        overlap)*    -   otherwise, the result is MAYBE (the area overlaps with the        larger but not the smaller target cell circle)

In the area case (scenarios 2 & 4), sub-case “b” (complete overlap)

Step S8.4 c. Is the Current Location Area Completely within the Area ofthe Target cell?

-   -   if s₁>d+l₀ then the result is YES (the smaller originating        circle is within the smaller target circle)*    -   if l₁<d+l₀ then the result is NO (the larger originating circle        goes outside the larger target circle)*    -   otherwise, the result is MAYBE (the area overlaps with the        larger but not the smaller target cell circle)

As previously mentioned, this example of the method is specifically forthe cell approximation that comprises a small circle and a large circle(L′=L). However, it generally applicable to all alternatives. Forinstance, the lines marked with an asterix (*) would only need slightmodification for the approximation to a small circle and a largerectangle option (L′=R). In this case, each step involving a largercircle would need evaluation again the rectangular parameters instead ofthe circular (e.g. 2 step analysis of x and y distances as in priorart).

The algorithms can be refined to interchange various parameters (e.g.use s₀ instead of l₀ in step 4) depending on the use case.

The MAYBE result can be ignored or swapped for YES or NO depending onthe use case. One embodiment would be to use the MAYBE result to prompta more detailed calculation (e.g. true elliptical) which occurs lessfrequently, or over a longer time, than the calculations describedabove.

Many modifications and variations to the described system are possible.For example, whilst the circles L, S for the cell approximation areconcentric in the described examples, they need not be and nonconcentric circles may more accurately describe cells where fillertransmitters are used to enhance cell coverage. Moreover, different cellapproximations may be used for inclusion in the cell selection process,and different cell approximations may be used for the originating celland the target cell. As another example, some embodiments may benefitmore from the use of a polar (radius, angle) based co-ordinate systemthan Cartesian.

1. A method of approximating cell geometry corresponding to a cellcoverage area in a cellular transmission system, comprising providingdata corresponding to first and second circular parameters for thecoverage area of the cell.
 2. A method according to claim 1 includingproviding said data as a function of major and minor axial extents of anellipse.
 3. A method according to claim 1 including providing said dataas a function of characteristics of relatively large and small circles.4. A method according to claim 3 including providing said data as afunction of characteristics of relatively large and small circles thatare concentric.
 5. A method according to claim 3 including providingdata corresponding to the centers of the circles.
 6. A method accordingto claim 1 including converting information corresponding to arectangular approximation of the cell into said data.
 7. A methodaccording to claim 6 wherein the rectangular cell information issupplied in terms of latitude and longitude.
 8. A method according toclaim 7 including converting said information into said data in adifferent reference frame.
 9. A method according to claim 7 wherein therectangular cell information is supplied by DVB-T SI (ServiceInformation), and including converting said information into a Cartesianreference frame.
 10. User equipment for use in a cellular transmissionsystem, comprising a processor configuration to provide datacorresponding to first and second circular parameters for thedimensional extent of at least one cell of the system.
 11. Userequipment according to claim 10 wherein the processor configuration isoperable to provide said data as a function of major and minor axialextents of an ellipse.
 12. User equipment according to claim 10 whereinthe processor configuration is operable to provide said data as afunction of characteristics of relatively large and small circles. 13.User equipment according to claim 12 wherein the processor configurationis operable to provide data corresponding to the centers of the circles.14. User equipment according to claim 10 wherein the processorconfiguration is operable to convert information corresponding to arectangular approximation of the cell into said data.
 15. User equipmentaccording to claim 14 wherein the rectangular cell information issupplied by DVB-T SI-information, and the processor configuration isoperable to convert said information into a Cartesian reference frame.16. User equipment according to claim 15 comprising a mobile deviceoperable to receive DVB transmissions.
 17. User equipment according toclaim 16 further operable as telecommunications apparatus.
 18. Userequipment according to claim 10 including circuitry to provide datacorresponding to its current location, and a processor to compare thecurrent location data with the data corresponding to the cell fordetermining whether a cell handover is to be carried out.
 19. Userequipment according to claim 10 wherein the processor is operable toselect one of a plurality of different approximate geometricalconfigurations for the cell in dependence on the relationship betweenthe values of said parameters.
 20. A cellular transmission networkincluding user equipment, base stations for transmitting signals in acellular configuration to the user equipment, and a processorconfiguration to provide data corresponding to first and second circularparameters for the dimensional extent of at least one of thetransmission cells provided by the base stations.
 21. A method ofapproximating cell geometry in a cellular transmission system,comprising providing data corresponding to first and second parametersfor dimensional extents of the cell, and selecting one of a plurality ofdifferent approximate geometrical configurations for the cell independence on a relationship that is a function of the values of saidparameters.
 22. A method according to claim 21 including selecting anapproximation of an elliptical cell configuration based on saidparameters.
 23. A method according to claim 22 including approximatingthe elliptical cell configuration as relatively large and small circles.24. A method according to claim 22 including selecting between saidelliptical cell configuration and a rectangular cell configuration basedon the parameters.
 25. User equipment for use in a cellular transmissionsystem, comprising a processor configuration to provide datacorresponding to first and second parameters for dimensional extents ofthe cell, and to select one of a plurality of different approximategeometrical configurations for the cell in dependence on therelationship between the values of said parameters.
 26. User equipmentaccording to claim 25 wherein the processor configuration is operable toselect an approximation of an elliptical cell configuration based onsaid parameters.
 27. User equipment according to claim 25 wherein theprocessor configuration is operable to approximate the elliptical cellconfiguration as relatively large and small circles.
 28. User equipmentaccording to claim 25 wherein the processor configuration is operable toselect between an elliptical cell configuration and a rectangular cellconfiguration based on the parameters.
 29. User equipment according toclaim 25 including circuitry to provide data corresponding to itscurrent location, and a processor to compare the current location datawith the data corresponding to the selected cell configuration fordetermining whether a cell handover is to be carried out.